1. Field of the Invention
The present invention relates to a process for increasing production of light olefinic hydrocarbons from hydrocarbon feedstock by catalytic cracking, and more particularly to a process for increasing production of light olefinic hydrocarbons from hydrocarbon feedstock by catalytic cracking, which can increase the production of ethylene and propylene in an overall process by recycling ethane, propane and a C4-C5 fraction and variably controlling the production pathway of a C6+ fraction.
2. Description of the Related Art
Light olefins, such as ethylene and propylene, are widely used in the petroleum chemical industry. These light olefins are generally produced by the thermal cracking (steam cracking) of naphtha in the presence of steam. The reaction in the steam cracking technology is carried out at a high reaction temperature of 800-900° C. in a short residence time. Generally, by the steam cracking technology, produces various kinds of olefins which have a composition determined with a limited range.
Typical products of the steam cracking technology are ethylene and propylene, and, according to the circumstances of a process, a C4 olefin component is produced as a byproduct. However, the C4 olefin component consists of various isomers which requires a multistage complex separation process for the production thereof. Also, olefins having 5 carbon atoms or more have low economic value, and thus, are converted to saturated hydrocarbons by hydrogenation. In the steam cracking technology, the recycling of olefin components of 4 carbon atoms or more to a thermal cracking reactor entails no economic advantage because it causes a coke problem that shortens the production cycle of a process.
FIG. 1 shows a process diagram showing a process for producing light olefinic hydrocarbons from hydrocarbon feedstock by steam cracking (i.e., thermal cracking) according to the prior art.
As shown in FIG. 1, in the steam cracking process, heavy naphtha feedstock 31 is generally fed together with steam 32, acting as an aid, into a high-temperature thermal cracking reactor 1 where the feedstock is converted to a reaction product 33 containing olefins. The reaction product 33 is fed into a quench tower 2 where it is primarily separated, according to boiling point into, in the order of higher boiling point, fuel oil 37, pyrolysis gasoline 36 containing C5+ hydrocarbons as main components, dilution water 35 resulting from the condensation of steam used as a reaction aid, and gaseous product 34 containing C4− hydrocarbons as main components. The gaseous product 34 is passed through a compressor 3 and finally fed into a demethanizer 7, during which it is passed through unit processes, such as a splitter 4 and a low-temperature heat exchanger 5, in order to recover heat. Hydrogen and methane, which have the lowest boiling points in the overall process, are produced as a fraction 42. In a deethanizer 8, a C2 fraction 47 is separated to the top, and the C2 fraction is passed through a C2 hydrogenation reactor 9 and fed into a C2 splitter 10 where it is separated into ethane 49 and ethylene 50. The ethane 49 is partially converted to heavy olefins 51 in a recycling furnace 11 and then recycled to the quench tower 2. In a depropanizer 12, a C3 fraction 53 is separated to the top, and the C3 fraction is passed through C3 hydrogenation reactor 13 and fed into a C3 splitter 14 where it is separated into propane 55 and propylene 56. In a debutanizer 15, a C4 fraction 58 is separated to the top, and the C4 fraction is passed through a butadiene extraction unit 16, and MTBE (methyl tertiary butyl ether) unit 17, a C3 hydrogenation reactor 18, and a C4 splitter 19, during which it is separated into a butadiene 59, isobutylene 61, 1-butene 65, and C4 LPG 64, respectively. In a depentanizer 20, a C5 fraction 66 is separated to the top, and the C5 fraction is passed through a C5 hydrogenation reactor 21 to produce C5 LPG 67. In a deoctanizer 22, a C6-C8 fraction 69 is separated to the top, and the C6-C8 fraction is passed through a PGHT (pyrolysis gasoline hydrotreating unit) 23 to produce an aromatic fraction 70. In the bottom of the deoctanizer 22, a C9+ fraction 68 is produced.
Also, by a fluid catalytic cracking (FCC) process which is used to increase the added value of high-boiling point fractions in oil refining plants and produces gasoline as a main product, light olefinic hydrocarbons can be produced as byproducts. This FCC process is widely known in the art as catalytic cracking technology using a catalyst in the form of fine particles, which behaves like fluid when treated with steam. In the FCC process, a heavier fraction than naphtha or kerosene used in the present invention, such as vacuum residue, atmospheric residue or gaseous oil, is used as feedstock, and gasoline is mainly produced, rather than light olefins, and thus, light olefins are not effectively produced.
Typical chemical processes of producing these light olefins, such as ethylene and propylene, include steam cracking processes, FCC processes, and process for the catalytic cracking of light fractions. The typical compositions of reaction products from these processes are shown in Table 1.
TABLE 1Reaction productReaction productfrom process forfrom steamReaction productcatalytic crackingcracking processfrom FCC processof light fractionsMethane16.131.213.91Ethylene32.051.920.71Ethane2.910.78.93Propylene16.654.822.06Propylene0.350.73.04C410.949.18.97C55.711.17.81C6 or more14.1879.613.58Others1.080.90.99
The prior art relating to the light fraction-catalytic cracking processes of producing light olefinic hydrocarbons from hydrocarbon feedstock, and preferably naphtha or kerosene feedstock, by catalytic cracking, will now be described.
U.S. Pat. No. 6,307,117 discloses a method for separating a catalytic cracking product into an H2/C3 fraction and a C4+ fraction. Also disclosed is a method for separating the C4+ fraction into a C4 fraction, a C5-C8 fraction and a C9+ fraction. Also, a method for additionally cracking the C4+ fraction in a steam cracking reactor is disclosed. However, these methods cannot make the effective utilization of reaction products with sufficient consideration to the characteristics of the catalytic cracking reaction.
U.S. Pat. No. 6,576,805 discloses a process for recovering an H2/C3 fraction in a catalytic cracking process, but fails to suggest a process structure for an overall reaction product, and particularly, fails to suggest the effective utilization of a C4+ fraction.
U.S. Pat. No. 6,602,920 discloses a process structure of sequentially using thermal cracking, hydrogenation and catalytic cracking steps to produce light olefins from natural gas feedstock. However, this process structure cannot be used in the inventive catalytic cracking process that uses hydrocarbon feedstock, and preferably naphtha or kerosene feedstock.
As described above, although the development of catalysts for the light fraction-catalytic cracking processes for producing light olefinic hydrocarbons from hydrocarbon feedstock, and preferably naphtha or kerosene, by catalytic cracking, has been actively made, there has been no suggestion of an effective process structure for producing light olefins.